The present invention is a method and apparatus suitable for consistently rotating bitmap images that are represented anamorphically. Using a compact dot growth capability associated with the present invention, it is possible to rotate images rendered as high addressability anamorphic bitmaps to produce consistent output regardless of the rotation direction.
It is well-known that bitmap images can be rotated. However, high addressability bitmaps pose additional problems for well-known image processing operations--in particular rotation. For example, using a printing machine with a capability of storing or buffering scanned data prepared at a resolution of 600.times.600.times.2 spots/inch (spi) a corresponding output of 1200.times.600.times.1 spi can be generated. While such high resolution is desirable because of the inherent improvement in image quality at the higher anamorphic printing resolution, rotating image data stored at 1200.times.600.times.1 spi is burdensome and subject to inconsistencies depending upon the rotation direction--clockwise (CW) or counterclockwise (CCW).
Heretofore, a number of patents and publications have disclosed bitmap rotation schemes, the relevant portions of which may be briefly summarized as follows:
U.S. Pat. No. 5,359,706 to Sterling, issued Oct. 25, 1994 teaches image rotation using block transfers. A digital image is rotated clockwise by (1) dividing it into rectangular contiguous blocks, and (2) rearranging the blocks.
U.S. Pat. No. 5,124,692 to Sasson, issued Jun. 23, 1992, is directed to a method and apparatus for providing rotation of digital image data. The invention provides a framestore architecture that permits direct addressing of digital image data in order to generate rotated image and direct addressing at standard video sampling rates in order to generate rotated image data in real time
U.S. Pat. No. 4,975,977 to Kurosu et al., issued Dec. 4, 1990, is directed to a rotation processing method and apparatus therefor.
M. Denber, in "Fast Small-Angle Bitmap Rotation," Xerox Disclosure Journal Vol. 17, No. 5, Sep./Oct. 1992 discloses an improved algorithm for bit map rotations through small angles.
In accordance with the present invention, there is provided a printing system for rotating a high addressability binary bitmap received for printing, the bitmap including an anamorphic two-dimensional array of input image signals, where a first resolution along a first dimension exceeds a second resolution along a second dimension, the printing system comprising:
an encoder for encoding the input image signals to produce a bitmap image that is represented by a regular two-dimensional array of multi-bit image signals having a resolution in both dimensions that is equivalent to the second resolution; PA1 a first memory for storing the array of multi-bit image signals at the second resolution; PA1 a rotation circuit for rotating the multi-bit image signals stored in said first memory with respect to the orientation of the input image so as to produce a rotated image at the second resolution; PA1 a second memory for storing the image signals of the rotated image; PA1 a compact dot growth circuit, operating on the multi-bit image signals stored in said second memory, for generating output signals, each output signal generated in response to a gray multi-bit image signal having an exposure pulse positioned in response to the states of the multi-bit signal and adjacent multi-bit image signals; and PA1 an output engine for outputting an image representation in response to the the high addressability signals generated by said compact dot growth circuit. PA1 In accordance with another aspect of the present invention, there is provided a method, operating in a printing system for rotating a high addressability binary bitmap received for printing by the system, the bitmap including an anamorphic two-dimensional array of input image signals, where a first resolution along a dimension exceeds a second resolution along a second dimension, the method comprising the steps of: PA1 encoding the input image signals to produce a bitmap image that is represented by a regular two-dimensional array of multi-bit image signals having a resolution in both dimensions that is equivalent to the second resolution; PA1 storing the array of multi-bit image signals in a first memory at the second resolution; PA1 rotating the multi-bit image signals stored in the first memory in an increment of about 90.degree. with respect to the orientation of the input image so as to produce a rotated image at the second resolution; PA1 storing the image signals of the rotated image in a second memory; PA1 in response to the multi-bit image signals stored in the second memory, generating high addressability binary signals, wherein each high addressability signal generated in response to a multi-bit image signal has an exposure pulse positioned in response to the states of the multi-bit signal and adjacent multi-bit image signals; and PA1 outputting an image representation in response to the the high addressability signals generated by said compact dot growth circuit. PA1 encoding the N.times.M resolution image bitmap to produce a second, P-bit per pixel image bitmap at an M.times.M resolution; PA1 rotating the second bitmap; PA1 operating on the M.times.M rotated bitmap to identify those P-bit per pixel signals representing gray pixels therein and producing a third binary image bitmap, wherein exposure pulses for the third binary image bitmap are selectively repositioned within a pixel exposure period so that the pulses abut a pulse from an adjacent pixel period causing the resultant dot for the adjacent pixel period to enlarge upon development.
In accordance with yet another aspect of the present invention, there is provided a method, operating on a programmable computer, for rotating a binary input bitmap image in about 90.degree. increments, said input bitmap image representing binary image signals at an N.times.M resolution, where N=P.times.M and where P is an integer that is greater than 1, said method comprising the steps of:
One aspect of the invention is based on the observation of problems with rotation of high addressability bitmap images as, for example, in resolution enhanced printing systems. Rotation of images in such systems may lead to disparities when equivalent rotations are accomplished in different directions. For example, a 90.degree. clockwise rotation may result in an output print different from that generated by a 90.degree. counter-clockwise rotation. Such difficulties arise as a result of attempting to simply rotate an image that is represented with one resolution being greater than and an integer multiple of the other (e.g., 1200 spots per inch (spi) in one principal direction and 600 spi in the other). This invention is based on the discovery of a technique that alleviates these problems by enabling rotation of anamorphically represented bitmaps. This technique can be implemented, for example, by circuitry or operations that first encode the image to produce an easily rotatable image and also to accomplish compact dot growth (pixel repositioning) after rotation of an encoded image. A machine implementing the invention may also include a laser-based electronic printing system, wherein the laser beam intensity or exposure level may be modified via pulse width position modulation (PWPM) in response to the signals representing the rotated image. Preferably using morphological-like, template-matching techniques for compact dot growth, the present invention facilitates the identification of specific pixel structures within a rotated image and repositions the structures so as to appear in a consistent fashion when rendered by an lOT.